The subject matter of the present invention is a method for the modification of starch with organofunctional alkoxysilanes and/or alkylalkoxysilanes, in the following named as organosilanes in an aqueous medium. The modified starch obtained by this method contains the organosilane in bound form and can be used directly for known applications, but especially for the hydrophobation or oleophobation of cellulosic material.
It is known, from U.S. Pat. No. 3,398,015 for example, to produce self-supporting films from amylose and to improve the properties of these films by treatment with alkyl chlorosilanes.
It is furthermore known to use starch as a filler in polymers, in the case of urethane resins for example (see for example European Patent No. A20,058,921). In the case of other thermoplastics, too, such as polyolefins or even elastomers, starch is known as a filler (cf. Shell Polymers, Vol. 5, No. 3 (1981), pp 73 to 78). Starch derivatives, such as the ethers or esters, are also used as fillers. It has also been proposed to use starch derivatives formed with silicones as fillers; in this case, however, the starch is merely mixed with the silicone, so that it is only a mixture of starch and silicone that is obtained, in which the silicone is not bound to the starch.
When starch is used as a filler, the starch is to have both an oleophilic surface and reactive centers which improve incorporation into the polymer and adhesion thereto. For this purpose, organo functional alkoxysilanes and alkylalkoxysilanes are already used as adhesion mediating agents between polymers and inorganic fillers, the fillers being able also to be used as modified fillers. The modification is performed by reacting the fillers with the organosilane by bringing the filler into contact with the organosilane either directly or dissolved in a solvent. Additional reaction mediators or catalysts are not necessary in this reaction.
If, in analogy to this known modification of inorganic fillers for polymers, the attempt is made to modify starch in an aqueous medium in this manner with this known organosilane, the desired results are not obtained: the organosilanols present in aqueous solutions do not react with the starch, modification of the starch does not take place, and the organosilane contained in the solvent merely adheres superficially to the starch. This behavior is displayed both by organofunctional silanes and by alkoxy silanes.
In view of the necessity arising in practice of modifying starch in an aqueous medium, the problem therefore existed of finding a method for fixing organosilanols onto starch in an aqueous medium. The starch is then to have reactive centers which, on the basis of the organofunctional moieties, in the case of organofunctional silanes, are able to react in a known manner with functional groups of polymers, or which, on the basis for example of the alkyl moieties of organosilanes, can be made particularly hydrophobic, resulting in good dispersibility and uniform distribution of the starch in polymers.
For the accomplishment of this purpose, a method has now been found for the modification of starch with organosilanes, which is characterized by bringing starch into intimate contact with an aqueous solution of the organosilane in the presence of alkali aluminate or alkali hydroxides.
In the practice of the method of the invention, the organosilane is dissolved in water to form its hydrolyzate, the starch is suspended in this solution, and this suspension is treated with an alkali aluminate or an alkali hydroxide, and then the starch is kept in constant movement. It is desirable for the aluminate or the alkali hydroxide not to be added until the starch has been well distributed as a suspension in the solution of the organosilane hydrolyzate. Preferably, the suspension is stirred, to obtain a good intermixture. The reaction takes place even at room temperature. Heating, after the addition of the aluminate or alkali hydroxide, is possible, but in that case the temperature is not to reach the swelling temperature of the type of starch involved, if it is desired to recover the modified starch by filtration. In general, within a stirring time of about 2 hours the amount of silane corresponding to the batch has reacted with the starch to form the modified starch.
In this kind of procedure, the modified starch obtained in accordance with the invention is worked up by known methods by separating the starch and then washing it with water and drying it. If no impurities enter the starch during the process, the washing process can be omitted.
It is, however, also possible, in accordance with the invention, to perform the modification of the starch by transforming it to an aqueous solution at elevated temperature and treating this solution with an aqueous solution of the hydrolyzate of the organosilane and the aluminate or alkali hydroxide, as the case may be. The aluminate or alkali hydroxide is preferably used separately in the form of an aqueous solution. This solution is preferably added to the mixture after the silane hydrolyzate solution. The simultaneous addition of the silanol solution and the aluminate or alkali hydroxide solution to the starch solution, however, is also possible.
The temperature at which the silane hydrolyzate and the aluminate or alkali hydroxide are added is generally in the range of the temperature at which the particular starch becomes pasty. Heating above this temperature is not necessary. After the addition of these two solutions, the starch solution is vigorously stirred. It can be allowed to cool while being stirred. The cooled starch solution then already exhibits the desired effects.
It is also possible to concentrate the modified starch solution by evaporation and thus obtain a modified starch in powder form. The silane cannot be washed out of this starch with water. If, however, the modification is not performed in the presence of aluminate or alkali hydroxide, the silane can be washed virtually completely out of a silane-containing starch made in this manner.
Proof that the starch has reacted with the organosilane is generally obtained by determining the silicon content. It is assumed that, theoretically, no more than one mole of organosilane can react with each free hydroxyl group of the starch. In practice, however, much lesser amounts of organosilane will suffice to give the starch the properties mentioned above. It will suffice if, for each free hydroxyl group of the starch, only 0.001 mole of the organosilane is bound to the starch. Good results are obtained when the amount of organosilane bound to the starch amounts to only 0.003 mole per mole of free hydroxyl groups. By the method of the invention, however, considerably greater amounts of the silane can be bound to the starch, but then the silane yield is lower. Generally, less than 1 mole of the organosilane per mole of free hydroxyl group is bound to the starch.
For the reaction between the starch and the organosilane, preferably dilute aqueous solution sof the organosilane are used. The organosilane reacts with the starch out of solutions of as little as 0.5%. Generally, there are employed 1.0 to 10% aqueous solutions but higher concentrated solutions, e.g., of 50%, can also be used. When such more concentrated solutions are used a smaller percentage of the silane reacts with the starch than in the case of the dilute solutions.
The preparation of the aqueous organosilane solutions is performed in a manner known in itself. The calculated amount of the silane is stirred into water, and the corresponding hydrolyzate is formed. Turbidity may be produced for a short time, but after the hydrolyzate formation is complete it vanishes. In the case of neutrally reacting organosilanes and higher concentration, a catalytic amount of acid (e.g., hydrochloric acid) can be added to achieve more rapid hydrolysis.
The alkali aluminate is used preferably in the form of an aqueous solution. The concentration of the solution is of secondary importance, but excessively dilute solutions should not be used. Preferably the commercially common 50% aluminate solutions are diluted to 5 to 15% for the purpose of greater ease in proportioning them. Basically, it is also possible to use the concentrated solutions, or also to dissolve the solid in the aqueous suspension of starch and organosilane.
Sodium aluminate is the preferred aluminate.
The alkali hydroxide is preferably also used as an aqueous solution, the concentration being best between 5 and 20%. More dilute solutions can also be used, especially if aqueous solutions of the modified starch are to be obtained.
The term alkali hydroxides, as used in accordance with the invention, is to be understood to refer to the hydroxides of sodium and potassium. Sodium hydroxide is the preferred alkali hydroxide.
The amount of the alkali aluminate or alkali hydroxide that is to be used is to be such that there will be at least 0.3 mole of alkali aluminate or alkali hydroxide for each mole of organosilane. The preferred molar ratio is between 1:0.4 and 1:1.5. Even with higher molar ratios, up to 1:2, results according to the within invention are obtained.
It is also possible to perform the reaction between starch, organosilane and aluminate or alkali hydroxide in a virtually water-free medium. In this case the components are very intimately mixed together in suitable mixing or kneading units, preferably extruders. In this method of procedure, it is advantageous to use the organosilane as a concentrated aqueous solution. Operating in a virtually water-free medium also makes it possible to operate briefly at temperatures above the swelling temperature of the starch.
The modification of the starch by organosilanes is performed in accordance with the invention either with native starch or starch derivatives. The native starch can originate either from corn or grains, or from tubers or root crops. Thus, for example, wheat starch, cornstarch, potato starch or even rice starch can be used. The starch derivatives which can be modified in accordance with the invention are mainly esters and ethers of the above types of starch, as well as oxidized starches. Cationized starch can also be used.
The organosilanes which can be used as modifying agents in accordance with the invention include alkyl alkoxy silanes and organofunctional alkoxysilanes. They can be described by the following general formula: ##STR1## in which Y represents a substituted or unsubstituted amino group, or a moiety from the group H, --CH.sub.3, --Cl, --CH.dbd.CH.sub.2, --SH, ##STR2## or --O--C(O)--C(CH.sub.3).dbd.CH.sub.2, X is an alkoxy moiety with a maximum of 6 carbon atoms, which if desired, is interrupted by oxygen atoms, m can assume the values 0 or 1 or 2, and n the values 1 or 2 or 3, as well as 0 in the case in which Y=--CH.dbd.CH.sub.2. Substituents for the amino group can be alkyl moieties of 1 to 8 carbon atoms, aryl moieties, preferably the phenyl moiety, cycloalkyl moieties, as well as the group --CH.sub.2 --CH.sub.2 NH.sub.2. The amino group can also be in the form of a quaternary ammonium salt. Under these general formulas come alkylsilanes such as methyltrimethoxysilane, methyltriethoxysilane, propyltrimethoxysilane, propylmethyldimethoxysilane, propyltrimethoxysilane, propylmethyldiethoxysilane, butyltrimethoxysilane, and isobutyltrimethoxysilane. The following are given as examples of organofunctional silanes: vinyltrimethyoxysilane, vinyltriethoxysilane, vinyl-tris-methoxy-ethoxysilane, vinyl-tris-ethoxy-ethoxysilane vinylmethyldimethoxysilane, gamma-chloropropyltrimethoxysilane, gamma-chloropropylmethyldimethoxysilane, alpha-chloromethyldimethylmethoxysilane, gamma-mercaptopropyltrimethoxysilane, gamma-mercaptopropylmethyl dimethoxysilane, gamma-aminopropyltrimethoxysilane, gamma-aminopropyltriethoxysilane, gamma-aminopropylmethyldiethoxysilane, N-amino-ethylaminopropyltrimethoxysilane, aminophenyltrimethoxisilane, gamma-glycidyloxypropyltrimethoxysilane, and gamma-methacryloxypropyltrimethoxysilane.
The starch modified in accordance with the present invention is not wetted by cold water when alkyl silanes and a number of organofunctional silanes are used as modifying agents. It is nevertheless soluble in hot water.
In a great number of cases in which organosilanes are used as modifying agents, the starches modified in accordance with the invention have good free-flowing qualities, while unmodified starches have virtually none at all. This improvement is striking mainly in the case of potato starch, but even other kinds of starches show good free-flowing qualities when modified with alkylsilanes.
The aqueous solutions of the starches modified in accordance with the invention can have a greater viscosity than those of the pure starches, but this does not have any disadvantageous effect when fillers are stirred in or dispersed.
On the basis of the properties described above, the modified starches prepared by the method of the invention are suitable especially in those applications of starch in which native starches and the known starch derivatives do not meet existing requirements. Examples that can be given of such applications are the use of starches as binders for mineral fibers, as textile adjuvants, as sizes for various paper products, and as fillers for plastics.
In addition to these known applications of starch, the modified starches prepared in accordance with the invention can also be used for the hydrophobation and oleophobation of cellulosic materials such as paper, cardboard or wood. Especially suitable for this purpose are the aqueous solutions which are obtained by the reaction of starch-pastes with the organosilane and an alkali aluminate solution, since these solutions are very stable and are widely variable in their starch and organosilane concentration.